1. Field of the Invention
The present invention relates in general to the field of radio frequency identification and more specifically to radio frequency identification devices with separated antennas including embodiments with circuit-antenna overlay.
2. Description of the Related Art
Uses and applications of technologies to track objects, including people, products, and animals, are becoming more and more valuable. Hand counting and manual scanning of bar codes have long been used to identify objects and track their activity. However, manual identification processes have significant limitations in terms of tracking a large amount of objects in a short amount of time. Automated radio frequency identification (“RFID”) technology has been embraced as an answer to manual identification processes. RFID technology represents an automated identification method that stores and receives data using RFID devices often referred to as RFID tags or RFID transponders.
FIG. 1 depicts an exemplary RFID device 100. RFID device 100 is, in at least one embodiment, a transponder that includes a circuit 102. Circuit 102 includes a memory, processor, transmitter, and receiver (see FIG. 3). Circuit 102 is preferably a small, cost effective integrated circuit; although, circuit 102 can be assembled using discrete components. The RFID device 102 also includes an antenna 104 for receiving an input signal X and for transmitting an output signal Y. The memory of circuit 102 stores data, such as an identification code. The processor of circuit 102 detects the input signal X and processes the input signal X by, for example, demodulating the incoming signal X, and modulating an output signal Y that is responsive to the input signal. The output signal Y generally includes the identification code of the RFID device 100 and may also include other data. The transmitter of circuit 102 transmits the data to a receiver by providing electrical signals to antenna 104. Antenna 104 converts the electrical signals into electro-magnetic waves.
The frequency(ies) of the input and output signals are in the radio frequency spectrum. The radio frequency (“RF”) spectrum encompasses frequencies in which electromagnetic waves can be generated by alternating current. In at least one embodiment, the RF spectrum ranges from 0+Hz to approximately 300 GHz. Commonly used frequencies for input and output signals are in the ultrahigh frequency range, i.e. 300 MHz-3,000 MHz and are generally modulated to 864 MHz-928 MHz or lower at 13.56 MHz.
RFID devices are categorized as “passive”, “semi-passive”, and “active”. Passive RFID devices have no internal power supply. The small amount of electrical current induced in the antenna 104 by the input signal X provides power for operation of circuit 102. A semi-passive RFID device includes a battery to supply internal power to the circuit 102 but not to transmit the output signal Y. An active RFID device includes an internal power source, such as a battery, that powers the circuit 102 and provides power for transmitting the output signal Y.
Multiple antenna designs exist for RFID devices. The antenna design generally depends on the type of RFID device and the signal frequencies. Antenna 104 represents one antenna design for a passive type RFID device. The antenna 104 extends from both sides of circuit 102. The pattern of antenna 104 is a matter of design choice. Antenna 104 includes a pattern with multiple, square, undulating paths. Antennas can be made using any of a variety of processes, including lithographic processes and wrapped wire. The circuit 102 can be connected to the antenna 104 using any connection technique.
RFID devices can be internally or externally attached to an object. For example, RFID devices can be attached to products, books, passports, transportation passes, vehicles, and animals. For example, RFID devices can be attached to packages to facilitate supply chain management. RFID devices can also be worn by humans, either externally or internally. For example, human worn RFID devices can be used to verify entrance authorization to a particular area or used to track an individuals attendance at different events.
FIG. 2 depicts an RFID portal 200 that detects RFID devices, such as RFID device 100, as the RFID devices pass within proximity of the antennas of RFID portal. RFID portal 200 is, for example, a DC400 from Symbol Technologies, Inc. a subsidiary of Motorola, Inc. with offices in Oakland, Calif. (“Symbol”). The RFID portal 200 includes two readers 206 and 208. Reader 206 includes RFID portal receiver RX 204a, receiver antenna 210, transmitter TX 202a, and transmitter antenna 212. Reader 208 includes receiver antenna 214 connected to receiver RX 204b and transmitter antenna 216 connected to transmitter TX 202b (connections not shown). The receivers RX 204a and 204b and transmitters TX 202a and 202b are coupled to processor 218. Processor 218 performs various processing function such as modulating an output signal X, demodulating an input signal Y, and retrieving the data transmitted by RFID tag 100. In one embodiment, processor 218 is an XR400 available from Symbol. The transmitter 202 transmits signal X with, for example, 1 watt of power. RFID device 100 responds to the transmitted signal X by transmitting signal Y to receiver 204. The power of signal Y is generally much smaller than the power or signal X and is, for example, approximately 1 microwatt. The maximum effective range (“MER”) represents a maximum distance between receiver or transmitter antennas and the RFID device over which the RFID device can be reliably identified by the receiver. The MER varies depending upon, for example, the type of RFID device and the medium upon which the RFID device is placed. Active RFID devices can have an MER of hundreds of meters. The MER of a passive RFID device varies from about 25 centimeters (cm) to a few meters.
The MER of passive RFID devices worn by humans is typically approximately 25 cm. Humans absorb RF radiation at the frequencies used for passive RFID devices and, thus, significantly attenuate the amount of energy available to power the circuit 102 in passive RFID device 100. To effectively gather data from a human-worn, passive RFID device, the wearer must pass within approximately 25 cm of the RFID portal antennas. Generally, positioning the RFID device of the wearer within such close proximity of the RFID portal transmitter and receiver is intrusive to the wearer because the positioning requires conscious effort by the wearer and slows the progress of the wearer. Thus, gathering information using RFID devices for humans is often unreliable due to mispositioning of the RFID device and slow due to the demands of close positioning.
FIG. 3 depicts a block diagram of circuit 300, which is one example circuit 102. Circuit 300 includes a transmitter 302 for transmitting output signal Y and a memory 304 for storing data, such as an identification code. Circuit 300 also includes a receiver 308 for receiving input signal X and a processor 306 for demodulating input signal X, retrieving data from memory 304 for transmission and modulating output signal Y. In at least one embodiment, the circuit 300 is an integrated circuit that may combine the functions of the transmitter, receiver, processor, and memory. In at least one embodiment, circuit 300 is an integrated circuit with length, width, and height dimensions which vary from, for example, 1-3 mm. Circuit 300 is, for example, EPC Global Gen 2 Higgs Chip available from Alien Technologies Corporation with headquarters in Morgan Hill, Calif., U.S.A.